Many arrangements have been previously used or proposed for the charging of various types of batteries including nickel-cadmium (NIcad) batteries which are quite capable of being charged at a rapid rate but which are easily damaged or destroyed if a high charge current is continued after a full charge condition has been reached. Chargers are commercially available which include mechanical timers set by the users to terminate charging after a set elapsed time. If set too long, dangerous overcharging may result. If set too short, the battery will not be fully charged. Other prior art arrangements have circuits designed for automatic control of termination of charge. For example, the Kosmin U.S. Pat. No. 3,938,021 discloses an analog circuit in which a series connected diode and capacitor are connected to a resistive voltage divider which is connected across terminals of a constant charge current source, an amplifier being provided having a high impedance input connected across the diode and having an output for controlling a transistor amplifier to turn off charge current when the charge voltage drops to a value such that there is no voltage across the diode. This type of circuit may operate satisfactorily but is not completely reliable and may allow charging beyond the full charge condition so that damage to batteries may result, in the absence of operator intervention.
A number of other prior patents contain proposals for use of digital or microprocessor circuits in charging or monitoring of the charge of NIcad or other types of batteries. These patents include the Ehlers U.S. Pat. No. 3,786,343, the Long U.S. Pat. No. 3,794,905, the Melling et al U.S. Pat. No. 3,890,556, the Melling et al U.S. Pat. No. 3,936,718, the Brandwein et al U.S. Pat. No. 3,940,679, the Siekierski et al U.S. Pat. No. 4,118,661, the Nicholls U.S. Pat. No. 4,191,918, the DeLuca et al U.S. Pat. No. 4,238,721, the Aspinwall et al U.S. Pat. No. 4,385,269, the Saar et al U.S. Pat. No. 4,388,582, the Saar et al U.S. Pat. No. 4,392,101, the Bollinger U.S. Pat. No. 4,418,310, the Koenck U.S. Pat. No. 4,455,523 and the Taylor U.S. Pat. No. 4,549,127. In general, such proposals involve the use of circuits which are complex and expensive and would not produce optimum results in charging of NIcad batteries.
An important consideration relates to the criteria used for determining when a charging operation should be terminated. The criteria which are disclosed in the aforementioned patents include the peak voltage condition, an inflection point of the voltage curve prior to the peak voltage condition, responses of a battery at certain times after application of a charging pulse and values or changes of temperature or other conditions. Peak voltage is used in the system of the Kosmin patent, a voltage proportional to peak voltage being stored by a capacitor for use in determining when the battery voltage has reached a peak and then dropped therebelow.
An attempt to use peak voltage as a criterion is also disclosed in the Siekierski et al patent, which describes the conversion of the battery voltage to digital data which is stored for comparison with new digital data developed later from conversion of the analog sum of the battery voltage and a predetermined reference increment which is introduced to avoid the effects of "jitter". The charge is terminated if the digital data derived from conversion of the analog sum are less than the stored data. In this system, the resolution of conversion, in volts per digital bit, must be greater than the value in volts of the predetermined reference increment which, in turn, must be greater than the magnitude of voltage variations or "jitter".
The Saar et al patents illustrate systems in which the inflection point is used as a criterion and the Taylor et al patent illustrates a system in which the criterion is the ratio of voltage differences developed in two periods following a charging pulse.